In the production of oil from subsurface formations, it is often necessary to stimulate production by hydraulically fracturing the formations with various mixtures. These mixtures may contain any number of ingredients including selected portions of sand, polymers and other chemicals added to water to form a slurry mixture for injection into the well. The hydraulic pressure applied to such formations creates tensile stresses in the rock of the formation resulting in fracturing or parting of the rock. These fractures are then filled by the injection of slurries containing propping agents under high pressure. Upon release of the pressure, the propping agent deposited in the fracture prevents the fractures from closing leaving channels through which oil and oil bearing fluids may flow to the well bore.
The concentration of the propping agent in the slurry is critical in that insufficient propping agent may not sufficiently maintain the fractures open and thus not provide full flow of oil and oil bearing fluids to the well bore. Use of excessive propping agent in the slurry may hinder the movement of the mixture into the formation and thus limit its effectiveness. Although devices have been designed to increase the concentration of the propping agent in fracturing fluid, including those disclosed in U.S. Pat. No. 4,126,181 to Black, and U.S. Pat. No. 4,354,552 to Zingg, these systems have not been well adapted to use where high fluid velocities are encountered in the proppant concentration process.
Thus, the need has arisen for an apparatus for concentrating the level of proppant in a fracturing fluid where such fracturing fluid is being injected into the well at high pressure and where high fluid velocities are encountered in the concentration of proppant in the fluid.